Silicon is a promising high capacity anode material for Li ion batteries. However, the large volume fluctuation upon Li+ insertion/extraction can fracture the material, leading to fast capacity fading due to the loss of electrical continuity. Another problem is that cracking exposes new surface of Si to the electrolyte solvents, which can decompose at low potential to deposit a solid electrolyte interface (SEI) layer of lithiated compounds on the new Si surface. During charge/discharge cycling, the insulating SEI layer can grow thicker, which further degrades the capacity and cycling stability of the Si anode. In an operating battery cell, continuous growth of SEI layer will also gradually deplete the available Li+ and the amount of electrolytes, thus deteriorating the overall performance.
Theoretical and in-situ transmission electron microscopy (TEM) studies have shown that the strain induced by the expansion/contraction can be accommodated in Si nanoparticles with diameters <150 nm. Indeed, it has been shown that various Si nanostructures including nanowires, nanotubes, hollow spheres, nanoparticles and nanoporous Si can withstand Li+ insertion/removal without significant cracking or fracture. However, the formation of SEI layers on these bare Si nanostructures limits their coulombic efficiency to <99% even after reaching steady state, which can drain the cathode and electrolyte in only tens of cycles. In comparison, the coulombic efficiency of graphite anodes can readily reach 99.9% after the first few cycles. One way to prevent the deposition of SEI on Si is to avoid its direct contact with the electrolyte solvent by applying a surface coating, which needs to be electrically conducting and permeable to Li+. Carbon based materials have been used for this purpose. (See, Yoshio, M.; Wang, H. Y.; Fukuda, K.; Umeno, T.; Dimov, N.; Ogumi, Z., Carbon-Coated Si as a Lithium-Ion Battery Anode Material. J. Electrochem. Soc. 2002, 149, A1598-A1603; Zhao, X.; Hayner, C. M.; Kung, M. C.; Kung, H. H., In-Plane Vacancy-Enabled High-Power Si-Graphene Composite Electrode for Lithium-Ion Batteries. Adv. Energy Mater. 2011, 1, 1079-1084; and He, Y. S.; Gao, P. F.; Chen, J.; Yang, X. W.; Liao, X. Z.; Yang, J.; Ma, Z. F., A Novel Bath Lily-Like Graphene Sheet-Wrapped Nano-Si Composite as a High Performance Anode Material for Li-Ion Batteries. RSC Adv. 2011, 1, 958-960.) However, a conformal carbon coating on Si would rupture upon volume expansion, exposing Si to electrolytes for SEI deposition. Therefore, carbon coatings that can accommodate the large volume expansion/contraction of Si are needed. This can be achieved by introducing void space between Si and its carbon coating. For example, very recently Liu et al., reported a yolkshell design of carbon encapsulated Si with high coulombic efficiency up to 99.84% from cycle 500 to 1000 (See, Liu, N.; Wu, H.; McDowell, M. T.; Yao, Y.; Wang, C.; Cui, Y., A Yolk-Shell Design for Stabilized and Scalable Li-Ion Battery Alloy Anodes. Nano Lett. 2012, DOI: 10.1021/n13014814.) Their approach was to first partially oxidize the Si nanoparticles to form a SiO2 surface layer and then form a thin shell coating of polymer, which was later pyrolyzed to amorphous carbon. Upon HF etching to remove SiO2 and reduce the size of the Si nanoparticles, void space was created inside the carbon hollow spheres that can accommodate volume expansion of Si during lithiation, thus preventing the rupture of the carbon shell and resulting in much improved cycling stability.